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mysql_affected_rows() returns the number of rows affected by the query, but the meaning of "rows affected" depends on the type of query. For INSERT, REPLACE, or DELETE, it is the number of rows inserted, replaced, or deleted. For UPDATE, it is the number of rows updated, which means the number of rows that MySQL actually modified. MySQL does not update a row if its contents are the same as what you're updating it to. This means that although a row might be selected for updating (by the WHERE clause of the UPDATE statement), it might not actually be changed.
This meaning of "rows affected" for UPDATE actually is something of a controversial point because some people want it to mean "rows matched"—that is, the number of rows selected for updating, even if the update operation doesn't actually change their values. If your application requires such a meaning, you can request that behavior when you connect to the server by passing a value of CLIENT_FOUND_ROWS in the flags parameter to mysql_real_connect().
Handling Queries That Return a Result Set
Queries that return data do so in the form of a result set that you deal with after issuing the query by calling mysql_query() or mysql_real_query(). It's important to realize that in MySQL, SELECT is not the only statement that returns rows. Statements such as SHOW, DESCRIBE, EXPLAIN, and CHECK TABLE do so as well. For all of these statements, you must perform additional row-handling processing after issuing the query.
Handling a result set involves the following steps:
Generate the result set by calling mysql_store_result() or mysql_use_result(). These functions return a MYSQL_RES pointer for success or NULL for failure. Later, we'll go over the differences between mysql_store_result() and mysql_use_result(), as well as the conditions under which you would choose one over the other. For now, our examples use mysql_store_result(), which retrieves the rows from the server immediately and stores them in the client.
Call mysql_fetch_row() for each row of the result set. This function returns a MYSQL_ROW value or NULL when there are no more rows. A MYSQL_ROW value is a pointer to an array of strings representing the values for each column in the row. What you do with the row depends on your application. You might simply print the column values, perform some statistical calculation on them, or do something else altogether.
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